Rotating shaft vibration monitoring systems are widely used to determine excessive shaft vibration that can lead to destruction of the machinery, and to provide information on which the rotating shaft or machinery can be balanced to reduce or eliminate the vibration. The vibration monitoring systems normally use transducers to detect the extent of the vibration. These transducers may comprise velocity pick-ups, accelerometers, and non-contacting transducer probes or proximity probes.
Eddy current proximity probes have proved to be the best transducers for machinery protection devices. However, the non-contacting eddy current displacement transducers suffer from the problem of "electrical runout". "Runout" is the mechanical imperfection in machine shafts. It has also been adopted as the name for the output signal of a vibration pick-up that does not represent shaft vibratory motion, but is caused by the eccentricity of the shaft, surface irregularities, or by properties of the shaft that cause the vibration probe to give an incorrect vibrating signal. This latter type incorrect signal is called "electrical runout" and it is the electrical runout that presents the major difficulties, because if the electrical runout is sufficiently large, it makes it difficult or impossible to accurately determine the extent of the vibrating signal in the composite output signal from the eddy current probe.
Mechanical and/or electrical runout can be reduced by proper finishing of the surface that the vibration pick-up is electrically observing, and methods such as shaft peening and knurling have been developed to reduce the electrical runout. The peening and knurling techniques, however, have only had limited success and do not help at all in the case of chrome plated shafts or shafts that are chrome-nickel alloy. While electronic elimination of the runout signal is used in some manufacturer's test stands, until the present, these methods are not suitable for use in plant monitoring systems due to the high cost per channel and the poor reliability of the continuous monitoring. It is therefore advantageous to have a low cost, reliable electronic runout subtractor that is suitable for use in standard, continuous, monitoring systems.